Overall Heat Transfer Coefficient (U)

\frac{1}{U} = \frac{1}{h_i} + \frac{L}{k} + \frac{1}{h_o}

Worked example: hi 5000, 3 mm steel k=45, ho 2000 → U = 1304.3 W/(m2.K) — press Try an example to run it live, then adjust anything.

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Overall Heat Transfer Coefficient (U) explained

Resistances in series add, and heat transfer borrows the electrical analogy wholesale: 1/U is the total resistance per unit area, the sum of the inside film, the metal, and the outside film. What falls out is the engineer's most useful insight — the largest resistance owns the answer. Take a steel exchanger tube, 3 mm of steel at k = 45 W/(m·K), water inside at hᵢ = 5000 and air outside at hₒ = 2000: 1/U = 0.000200 + 0.0000667 + 0.000500 = 0.000767, so U = 1304 W/(m²·K). Note that the steel — the only part you can see — contributes 9% of the resistance. Doubling the tube wall barely moves U; doubling the air-side film nearly doubles it.

That is why finned tubes exist. Air-side coefficients are typically 20–50 times worse than water-side ones, so every gas-to-liquid exchanger in the world grows fins on the gas side to buy back area rather than coefficient. The trap on real datasheets is area basis: U must be quoted against a stated area (usually the outside), and a U of 850 W/(m²·K) on inside area is not the same machine as 850 on outside area. When you see U and A on a drawing, check which surface A refers to before you believe the duty.

Overall Heat Transfer Coefficient (U) formula

\frac{1}{U} = \frac{1}{h_i} + \frac{L}{k} + \frac{1}{h_o}

Where

$U$ = Overall coefficient (W/(m²·K))
$h_i$ = Inside film coefficient (W/(m²·K))
$L$ = Wall thickness (mm)
$k$ = Wall thermal conductivity (W/(m·K))
$h_o$ = Outside film coefficient (W/(m²·K))

Missing one of these? Work it out first, then come back

Overall coefficient — Overall U from Total Resistance, Heat Exchanger Duty (Q = U·A·F·LMTD)
Inside film coefficient — Fin Parameter mL (Straight Fin), Fin Heat Transfer Rate
Wall thickness — Thermal Resistance of a Plane Wall, R-Value of a Snow Wall
Wall thermal conductivity — Thermal Resistance of a Plane Wall, Conduction Through a Pipe Wall
Outside film coefficient — Critical Radius of Insulation, Fin Parameter mL (Straight Fin)